A solid oxide fuel cell (Solid Oxide Fuel Cell: hereinafter referred to as an SOFC as necessary) is characterized in that an oxide ion (O2−) conductor is used as a solid electrolyte and is made up of a fuel electrode and an oxygen electrode, disposed in such a way as to sandwich a solid oxide electrolyte therebetween. At the time of operation, an electrochemical reaction is caused to occur by feeding fuel toward the fuel electrode and an oxidizing agent toward the oxygen electrode, thereby extracting electric power. As the oxidizing agent fed toward the oxygen electrode, use is made of oxygen-enriched air, oxygen, etc. besides air, however, the description will be given hereinafter assuming that the oxygen electrode is an air electrode and the oxidizing agent is air as appropriate.
FIG. 1 is a schematic illustration showing a disposition relation among a solid oxide electrolyte, a fuel electrode and an air electrode, making up a single SOFC, and the operation principle thereof. The single cell is made up such that the fuel electrode and air electrode are disposed with the solid oxide electrolyte interposed therebetween. Oxygen contained in air guided to the air electrode is turned into oxide ions (O2−), reaching the fuel electrode after passing through the solid oxide electrolyte. Hereupon, the oxide ions (O2−) react with a fuel, such as hydrogen, and so forth, guided to the fuel electrode, to release electrons, thereby producing reaction products such as electricity, water, and so forth. Air after use at the air electrode is discharged as an off-gas of the air electrode and fuel after use at the fuel electrode is discharged as an off-gas of the fuel electrode. In the present specification, the off-gas of the air electrode is referred to as discharged air and the off-gas of the fuel electrode is referred to as discharged fuel.
FIGS. 2 and 3 are schematic illustrations showing a mode of a flat plate type SOFC, respectively, by way of example. The SOFC includes a cylindrical type, an integrated stacked type, and so forth; however, these types are the same in principle as the flat plate type SOFC. FIG. 2 is the schematic illustration showing a self-supporting membrane type SOFC while FIG. 3 is the schematic illustration showing a supported membrane type SOFC. In FIGS. 2 and 3, there is shown a single cell, respectively, however, since a voltage of one unit of the single cell is low, the single cells are normally stacked one on top of another in a plurality of layers. In the present specification and drawings, a structure made up by stacking the single cells one on top of another in a plurality of layers is referred to as a solid oxide fuel cell stack (=an SOFC stack) or simply as a stack.
In the case of the self-supporting membrane type SOFC, as shown in FIG. 2, a sealant 2 and a separator 3 are sequentially disposed on top of a single cell 1 while a separator 4, a sealant 5, and a separator 6 are sequentially disposed underneath the single cell 1, these component members being closely stacked to thereby make up the self-supporting membrane type SOFC. For the sealants 2, 5, use is made of a ceramic material such as glass, or the like, and for the separators 3, 6, use is made of a ceramic material such as MgAl2O4—MgO, or the like, while for the separators 4, use is made of a ceramic material such as lanthanum chromite, and so forth.
In the case of the self-supporting membrane type SOFC, the structure is retained by a solid oxide electrolyte membrane by itself. Accordingly, the solid oxide electrolyte membrane is required to have a large film thickness thereof and is normally formed to a thickness on the order of 100 μm. Further, the operational temperature thereof is in a range of 800° C. to 1000° C., normally as high as about 1000° C., forming a high temperature field in excess of 1000° C. inside the stack. The component members thereof are therefore limited to expensive heat-resistant alloys or ceramics. In the case of the ceramics, there arise such problems as cracks occurring thereto due to differences in temperature occurring inside the stack, and so forth, while in the case of housing the stack in an adiabatic vessel, a thermal insulating material of the adiabatic vessel increases in thickness, resulting in an increase in the size of an SOFC system.
On the other hand, in the case of the supported membrane type SOFC, as shown in FIG. 3, a single cell 7 is retained inside the frame of a spacer 8, and on top of the single cell 7, there are sequentially disposed a bonding material 9, a cell supporter 10, a spacer 11, and a separator 12. Although there are also sequentially disposed a bonding material, cell supporter, spacer, and separator underneath the single cell 7, these component members are omitted in FIG. 3. In the case of the supported membrane type SOFC, a solid oxide electrolyte membrane is made up by use of, for example, a LaGaO3-based or zirconia-based material, being formed to have a film thickness as small as, for example, on the order of 10 μm, so as to be supported by a fuel electrode large in film thickness.
The inventor et al. have focused attention on the supported membrane type SOFC, in particular, and continued efforts for the development thereof, having thus far obtained several successful results (JP-A 2002-367615). With the supported membrane type, since, for example, the solid oxide electrolyte membrane can be formed to a small film thickness, it is possible to perform operations at a low temperature, lower than the temperature in the case of the self-supporting membrane type, that is, in a range of 650° C. to 850° C., for example, at as low as 750° C. As a result, for the constituent material of the spacer 8, bonding material 9, cell supporter 10, spacer 11, separator 12, and so forth, respectively, use can be made of an inexpensive material such as stainless steel, for example, ferritic stainless steel etc. and furthermore, the supported membrane type SOFC has various other advantages including implementation of a reduction in size.
Incidentally, with the SOFC, carbon monoxide as well is used as fuel besides hydrogen. Accordingly, in the case of using town gas, LP gas, etc., containing hydrocarbons as constituents, for raw fuel, such raw fuel is reformed so as to be converted into hydrogen and carbon monoxide before use. Methane, among hydrocarbons, is converted into hydrogen and carbon monoxide through internal reformation by the agency of catalysts such as Ni, and so forth, provided at the fuel electrode of the SOFC and they are used as the fuel of the SOFC.
However, if the raw fuel contains hydrocarbons other than methane, that is, hydrocarbons having two or more carbon atoms, such as ethane, ethylene, propane, butane, etc., this will cause carbon to be formed on piping to the SOFC and the fuel electrode, which blocks the electrochemical reaction, thereby resulting in deterioration in cell performance. These problems attributable to the hydrocarbons having two or more carbon atoms will have vital effects on the SOFC which is repeatedly operated for a long time period while in service.
Town gas, LP gas, natural gas, gasoline, or kerosene, etc. includes hydrocarbons other than methane, that is, hydrocarbons having two or more carbon atoms. For example, in the case of town gas, to give an example of the composition thereof, the same contains 88.5% methane, 4.6% ethane, 5.4% propane, 1.5% butane (herein % represents vol. %, the same applies hereinafter), so that about 11.5% of hydrocarbons having two to four carbon atoms are contained in addition to methane as the main constituent thereof. For this reason, in order to use those as the raw fuel of the SOFC, there is the need for reforming those hydrocarbons having two or more carbon atoms such that the hydrocarbons having two or more carbon atoms are removed by converting the same into methane, hydrogen, and carbon monoxide.
There are available the steam reforming process and partial combustion process as processes for reforming hydrocarbons. These processes represent techniques for converting hydrocarbons into a reformed gas with hydrogen as the main constituent thereof and, in the case of hydrocarbons being, for example, methane, the steam reformation reaction can be expressed by the following formula:CH4+H2O=3H2+COWith the SOFC, however, not only hydrogen and carbon monoxide, needless to say, but also methane as well after conversion into hydrogen and carbon monoxide, upon internal reformation at the fuel electrode, can be used as fuel, so that it needs only to be sufficient if the hydrocarbons having two or more carbon atoms are removed from the fuel in a stage of being fed to the fuel electrode of the SOFC, thus eliminating the need for reformation of the fuel up to a reformed gas with hydrogen and carbon monoxide as main constituents thereof.
Thus, for the reformation of the raw fuel for use in the SOFC, it will suffice to remove the hydrocarbons having two or more carbon atoms by conversion thereof into other constituents such as methane, hydrogen, carbon monoxide, etc., so that it is unnecessary to convert all the hydrocarbons into hydrogen, and carbon monoxide. Accordingly, with an SOFC system, use is made of a reformer for executing reformation whereby the hydrocarbons having two or more carbon atoms are converted into other constituents such as methane, hydrogen, carbon monoxide, etc. to be thereby removed from the raw fuel. That is, with the reformer of the raw fuel for use in the SOFC, there is no need for converting all the hydrocarbons into hydrogen, and carbon monoxide, and the hydrocarbons having two or more carbon atoms, contained in the raw fuel are converted into methane, hydrogen, carbon monoxide, etc., resulting in removal of the hydrocarbons having two or more carbon atoms.
In the present specification and drawings, a reformer used for this purpose is referred to as a preliminary reformer, and fuel prior to preliminary reformation with the use of the preliminary reformer is referred to as raw fuel.
FIG. 4 is a schematic illustration showing a system wherein the preliminary reformer according to the steam reforming process is disposed together with the SOFC stack, using the town gas, LP gas, and so forth as the raw fuel. As shown in FIG. 4, there are sequentially disposed a desulfurizer 13, a steam generator 14, a preliminary reformer 15, and a SOFC stack 16. There can be a case where the steam generator 14 is integrated with the preliminary reformer 15. If a sulfur compound is contained in the raw fuel, the fuel electrode of the SOFC will undergo poisoning caused by the sulfur compound. The town gas, and LP gas contain sulfur compounds such as mercaptan, and others, serving as an odorant, and natural gas, gasoline, etc. also contain sulfur compounds such as hydrogen sulfide, and so forth, although the content thereof varies depending on production sites, and so forth. The desulfurizer 13 is used for removal of the sulfur compounds from the raw fuel, but the desulfurizer 13 is unnecessary if the raw fuel does not contain sulfur or sulfur has already been removed. The raw fuel, after being desulfurized in the desulfurizer 13, is fed to the preliminary reformer 15 together with steam generated by the steam generator 14, whereupon the hydrocarbons having two or more carbon atoms are reformed to be converted into methane, hydrogen, carbon monoxide, and so forth. A reformed gas is guided to the fuel electrode of the SOFC stack 16.
Now, as described above, the operational temperature of the self-supporting membrane type SOFC is in the range of 800° C. to 1000° C., and is normally at as high as about 1000° C., and the operational temperature of the supported-membrane type SOFC is not higher than about 850° C., that is, lower than the operational temperature of the self-supporting membrane type SOFC, but is still high. For this reason, there is the need for reducing heat loss from the SOFC stack, the preliminary reformer, and so forth, as much as possible, and accordingly, it is considered appropriate to house those component equipment in the adiabatic vessel, or to cover the same with a thermal insulating material. In addition, in view of the need for retaining a constant temperature in the above-described range during the operation of the system, it is desirable to heat air and fuel to be guided to the SOFC before feeding the same.
It is therefore an object of the invention to provide an SOFC system wherein by combined use of an SOFC stack, a preliminary reformer for use in SOFCs, and an integrated heat exchanger for catalytic combustion for use in the SOFCs, constituting the SOFC system, respective advantages of both equipment, that is, the preliminary reformer and the integrated heat exchanger for catalytic combustion, are obtained, and heat loss of the SOFC system is eliminated or reduced as much as possible, and also to provide the integrated heat exchanger for catalytic combustion for use in the SOFC system.